Sonic the Hedgehog 2 v0.9.12

Correct native Emerald Hill scrolling for custom and mixed character parties.

The last two scroll rows now use the original Knuckles-in-Sonic-2 correction whenever the validated roster differs from solo Sonic, solo Tails or the canonical Sonic+Tails duo. This includes reversed pairs, custom characters and active extra companions. Unused seats preserve original classic-roster fidelity. Existing Knuckles-leading behavior, widescreen output and native VS scrolling are preserved.

The runtime engine and launcher code are unchanged; the engine pin includes a documentation refresh. Fresh installs retain adaptive widescreen Fit to screen; explicit saved choices and native in-game saves remain intact. Quicksaves are specific to the exact build. Supply your own supported ROM and optional donor images; no ROM data ships.

The existing very-long-pause watchdog defect remains tracked separately and is not claimed fixed here. Broader platform/layout/collision/rewind enhancements are outside this release.

Validation passed: 25 focused tests, 17 matched roster cases with 51 paired snapshots, 23 quickstate fixtures, 2,000 matched replay executions, 117 rollback checks, and special-stage warm/cold restoration and track-map checks. The exact archive passed its dependency/allowlist checks and shipped replay helper; launcher persistence and fresh/saved aspect choices were checked with actual gameplay output. Two-roster CPU-time comparisons were neutral/inconclusive within the observed variation.